Revenge of the Weeds

Plant pests are evolving to outsmart common herbicides, costing farmers crops and money.

By Amy Coombs | May 20, 2012

Corn fieldsWIKIMEDIA COMMONS, GARDENKITTY

It’s a story suited for a Hollywood horror film, yet it’s also a tenet of evolutionary biology. Introduce a toxin to a system, and you inevitably select for resistant survivors. These few individuals gain a reproductive advantage and multiply; sometimes they can’t be stopped with even the most potent chemicals.

For years, this general plot line made headlines in the fields of antibiotic resistance and cancer research. More recently, plants have become a common protagonist. Weeds around the world are developing resistance to glyphosate—one of the most common herbicides on the market—and like bacteria and tumor cells, many plants can also withstand multiple other toxins, each with unique molecular targets.

In January, a hair-raising infestation of the kochia shrub was confirmed in Alberta, Canada. Originally introduced to desert climates as forage for cattle, the tenacious weed can now survive glyphosate, which targets an enzyme involved in the biosynthesis of aromatic compounds. It can also withstand chemicals that inhibit the ALS enzyme, involved in the production of amino acids. At least 2,000 acres are now impacted, and “we expect more cases will be confirmed after a field survey this fall,” says Hugh Beckie of Agriculture and Agri-Food Canada, the government department that manages farming policies.

The United States are also being taken by storm. Palmer amaranth recently developed resistance to the same two classes of chemicals in Tennessee. Since 2009, the tall, spindly weed has swept across 1 million acres of cropland, causing some farmers to abandon their fields. And in California, a plant named hairy fleabane recently crept into vineyards. It is now able to withstand both glyphosate and Paraquat—a chemical that hijacks photons from proteins involved in photosynthesis.

Worldwide, 23 weed species have developed glyphosate resistance, and at least 10 of these have also developed resistance to other herbicides, according to the International Survey of Herbicide Resistant Weeds. And Bill Freese, of the Center for Food Safety in Washington, DC, believes these numbers underestimate the problem. In order for a weed to be listed as resistant, it must survive four times the concentration used to kill susceptible plants. “Some weeds tolerate lower levels of glyphosate, and these also have a big impact in the field,” he says.

Weed infestations are more of a nuisance than a monstrosity—but they are biting into farmer’s pocketbooks. In Arkansas, 61 percent of soybean fields are infested with glyphosate-resistant Palmer amaranth, costing farmers $71 million a year in lost yields, and 80 percent of the state’s cotton is also infested, with losses now totaling $10.9 million.

Selecting for super-weeds

It’s not uncommon for bacteria to multiply every 20 minutes, but plants have a much longer life cycle and thus a slower rate of evolution. This makes herbicide resistance deceptively improbable.

The chance that a single mutation will confer herbicide resistance is 1 in 100,000, making the likelihood of a double resistant mutant less than 1 in 10 billion. Early industry-sponsored research suggested resistance to glyphosate was particularly unlikely because large mutations in the herbicide’s target, the EPSPS enzyme, would render it dysfunctional, killing the plant before it could reproduce.

“The claims made were naïve, and resistant weeds have indeed developed,” says David Mortensen, a weed scientist at Pennsylvania State University. “When a chemical is applied to such a wide area—to nearly all soybean and cotton, and a big percentage of corn—the selection pressure is too intense.”

Indeed, glyphosate use has increased dramatically, from the 4 million or so pounds that were applied to corn in 2000 to 65 million pounds last year, with use on cotton and soy fields also climbing. Much of this increase can be attributed to the incorporation of genetically engineered crops that are unaffected by glyphosate, which is sold by St. Louis-based Monsanto under the brand name Roundup. To help farmers spray glyphosate directly over fields without harming crops, Monsanto released Roundup Ready soy and canola in 1996. Genetically engineered cotton and corn soon followed, and by 2001, the GE crops spanned millions of acres. This is when resistant weeds made their debut.

“Glyphosate has been around since the 1970s, but resistant weeds didn’t become a serious problem until the herbicide was packaged with genetically engineered crops,” says Mortensen.

But according to Rick Cole, who over sees weed management for Monsanto, over-use is not the problem, and the solution is simply to apply a second herbicide with a different molecular target. “The problem is that glyphosate is used alone—over and over again,” he says. “Using a second mode of action reduces the risk of resistance.”

In a set of recommendations released earlier this month (May 10), the Weed Science Society of America suggested that in addition to minimizing herbicide use, farmers also diversify the chemicals they apply to their plants in order to keep herbicides effective for the long term.

Yet combining chemicals doesn’t always work out as planned, says Patrick Tranel, a weed scientist at the University of Illinois. While doubling up on chemicals makes it unlikely that anything will survive, he says, it also “potentially increases the chance of selecting for a general mechanism that confers resistance to both herbicides.”

Masters of survival

Weeds can’t grow wings or run away, so in order to survive harsh environments, they have developed diverse, generalized adaptation responses. Recently, Washington University professor Andre d’Avignon stumbled onto a surprising example. Using nuclear magnetic resonance imaging, he and his team traced glyphosate through horseweed, a native plant that infiltrates row crops with windblown seeds. In susceptible weeds, the chemical ended up in the cytosol, but in resistant plants, 85 percent was bound up in the vacuole within 24 hours. This mechanism has also been demonstrated in glyphosate-resistant Italian ryegrass and Rigid ryegrass.

Conyza bonariensis (horseweed) seed heads and flowers

WIKIMEDIA COMMONS, RICKJPELLEG

“This is a more generic process than the alteration of an enzyme, and it means a single plant can likely adapt to two chemicals at once,” says Mortensen.

In bacteria, multiple drug resistance is often traced to the large, diverse class of ATP-binding cassette (ABC) transporters, which carry an array of chemicals into the cell and organelles. This same class of proteins may help resistant plants sequester glyphosate. Indeed, d’Avignon has found evidence that horseweed may over-express a localized vacuole transporter, and while the protein has yet to be isolated, unpublished results suggest it requires ATP and traffics other chemicals typical of ABC transporters. “If there is an ABC transporter involved, it could possibly transport more than one herbicide to the vacuole,” says d’Avignon, whose work is sponsored by Monsanto.

Another strategy—used by plants and bacteria, alike—is to overexpress targeted enzymes, so that some can still function properly even while others are destroyed by the chemical. Glyphosate-resistant waterhemp, Palmer amaranth, kochia, and Italian ryegrass, for example, all overexpress the EPSPS gene that the herbicide targets. Yet instead of simply upregulating gene expression, glyphosate-resistant plants make multiple copies of the EPSPS gene. “This is very hard to do—it’s much less likely than a point mutation,” says Tranel.

The rise of the resistant

If the situation wasn’t bad enough already, it appears to be snowballing. Weeds in nine different countries have independently developed resistance to multiple modes of action. Some stubborn survivors can now survive most of the chemicals used by farmers, and the infestations are spreading.

Last year, for example, farmers in Iowa reported infestations of waterhemp in their corn and soy fields. The weed has now encroached on 500 acres, and continues to survive treatments of glyphosate and six additional chemicals. The case is a rare example of a weed developing resistance to three chemical classes, each with a unique molecular target. Even more impressive, a biotype of Rigid Ryegrass growing in Victoria, Australia, is now resistant to four chemical classes. Only about 10 acres are impacted so far, but the weeds are predicted to spread.

Despite the seemingly small odds of a plant evolving resistance to multiple herbicides, the dramatic increase in glyphosate-resistant weeds, which now infest more than 17 million acres nationwide, has made this possibility exponentially more likely. “We don’t need a single plant to undergo two unlikely adaptations—we just need one event to happen in a biotype that already has glyphosate resistance,” says Mortensen.

The next wave of genetically modified seeds will be the ultimate experiment, says Freese. Monsanto is developing crops that can be simultaneously sprayed with glyphosate and the herbicide dicamba. Dow AgroSciences plans to market genetically engineered corn seeds resistant to both glyphosate and the herbicide 2,4-D; the US Department of Agriculture is reviewing the technology this summer. “Just as we had an increase in glyphosate [use] after Roundup Ready crops were released, we may soon see a huge increase in 2,4-D,” Freese says. “In our view, this will also lead to weeds with multiple resistance to 2,4-D and glyphosate. ”

Correction (May 24): The odds of a plant developing double resistance to the chemicals mentioned in this story are more like 1 in 10 billion. The original version stated "less than 1 in a trillion," which reflects a less conservative number. The Scientist regrets the error.

Correction (May 31): Sixty-one percent of soybean fields in Arkansas, not Alabama, are infested with glyphosate-resistant Palmer amaranth. The Scientist regrets the error.

Correction (August 7, 2014): Andre d’Avignon is at Wasington University, not the University of Washington. The Scientist regrets the error.

Weeds historically have adapted and grew stronger through their Darwinian desire to survive.Â Similar to protecting a "desirable" species in the wild and decimating it's predators.Â That's Mother Nature!

Some countries like Canada have wisely outlawed application of herbicides to lawns.Â People use these things indiscriminately.Â We need to put many more people back into agriculture, and alternate fallow ground that supports a diversity of predaceous insects and birds with crop land, the way it used to be done.Â In short, we need to live in harmony with nature.Â The only alternative (but clearly the one favored by chemical and agricultural industries) is to turn all of the surface of the Earth into a semi-sterile battleground.Â And, people have got to stop this lawn maintenance craze, that has no practical function whatsover, but deprives wildlife of its place on this planet.Â There are rational solutions, but can be ever get there when our politics is controlled by Corporations that have been granted the same rights as human beings by our Supreme Court?

Thanks for your interest! There is a lot of talk about whether the use of the word "evolution" is appropriate in all cases of resistance to glyphosate. Â And no one has characterized multiple resistant weeds to show what, exactly, is going on. Are enzyme targets and transporters being over-expressed (which could happen through a plastic response), or have mutations arisen in multiple pathways to create resistance to two or more modes of action? Â Of course plastic adaptations can revert--even after generations--when the environmental stimuli is no longer present. Â The research suggests there is likely a combination of factors at work, including mutation-driven trait development, over-expression through gene copies, and possibly plastic responses. Â Each case of resistance may be unique. Â This makes multiple resistance even more challenging to characterize and very difficult to describe in simple terms. Thanks for reading. -Amy Coombs

*Antibiotic-resistent bugs, herbicide-resistant weeds, what will be the next-resistant monster? *

This is also happening with vaccines as well.Â We have hepatitis B strains which started to be "selected" by vaccine pressure within a year of the vaccine being on the market, but people aren't told about that.Â The current world-wide whooping cough epidemic has nothing to do with "lack of us" but everything to do with new and more toxic strains developing from "pressure" from the vaccine - same as with weeds.Â Same with measles and other viruses. And often these "new" diseases show in atypical forms, and are much harder to diagnose, therefore easier to miss.Â But nothing of that is discussed, or else people would start asking if there is actually one area of science that doesn't have multiple skeletons rattling around in the cupboard.

Resistance to pesticides is nothing new. Resistance to insecticides, acaricides, herbicides and fungicides has been happening for decades. The massive challenge is to manage resistance to try to preserve the pesticides we have at our disposal. To that end, the use of mixtures of pesticides, the use of rotations of pesticides or the application of pesticides in mosaics have been suggested but, to date, there is more theory than actual testing and practice. More effective monitoring for resistance is also required. I'd suggest that the use of glyphosate in a mixture with another herbicide to which there is no resistance is a risky strategy and one that could only be employed where resistance to glyphosate does not already exist. Otherwise, the use of the mixture is likely to be no better than use of the second herbicide alone.

Herbicide resistance is nothing new - millions of hectares of specific weeds evolved (not "developed" as written in the article) resistance to the herbicide atrazine. Â The first book on the subject "Herbicide Resistance in Plants" was published in 1982 by Wiley, and others have been published over the years. Â That does not mean that the issue is not serious, as glyphosate is one of the best, cheapest, with least toxicological and environmental problems of herbicides developed to date, and industry has not found and commercialized a new mode of action herbicide in decades.

Amy, thank you for the enlightening article. Â Where should we look to find a solution? Â What direction should we be moving toward? Â I'm just getting into the industry a little bit with an organic company, Gantec Inc, and would benefit from solid expertise and guidance.

Why doesn't Mother Nature allow weeds to gain the upper hand with organic farming? Humans invented farming thousands of years ago. After all of that time, why is it that weeds have only become resistant in the past 20 years?

You need to provide a definition to the word "weeds"?Â Why do youÂ say "weeds" have only become resistant in the past 20 years?Â The term "weed" is objective, depends who is describing the organism.Â Also, as in other areas the spraying of other effects to the area's environment, ie. nesting areas, bees, butterfly population.Â

Without the use of the herbicides and pesticides in the past there would be no way of producing enough food for the world throughÂ so-called "organic" farming.

In my comment, I was trying to convey that some times through the use of herbicides and pesticides the weeds in question become immune and require more of said herbicides and pesticides to control the same "weeds"Â

Never mind the weeds, do we have any idea what this corn modified to absorb various poisons is doing to us?Â Just how much corn is soaked in weed killer? Â What good is a bumper crop of corn if it is lethal?

Glyphosate also kills the native plants that live alongrivers, and it kills the weeds monarch butterflies need forsurvival.Â There are also studies thatshow glyphosate kills good predatory insects, which makes pest problems a lotworse on farms.Â

Glyphosate also kills the native plants that live alongrivers and near farms, and it kills the weeds monarch butterflies need forsurvival.Â There are also studies thatshow glyphosate kills good predatory insects, which makes pest problems a lotworse.Â

Glyphosate also kills the native plants that live alongrivers and near farms, and it kills the weeds monarch butterflies need forsurvival.Â There are also studies thatshow glyphosate kills good predatory insects, which makes pest problems a lotworse.Â

Excerpts: "For whatever reason, dead and deformed piglets are a problem in all pig farming situations where GM soy is used in the diet. Mr Pedersen is convinced that this is connected to the residues of glyphosate that are allowed in feed within the EU..."Only two days after this danish pig farmer stopped using GM-Soy in the fodder for the pigs, allmost all the diarrhea disappeard. This dramatic change in the pig health alone justified the switch back to conventional fodder, the savings on the medicines justified the extra cost for the non-GMO fodder. Then add to this that the use of non-GMO also brought back normal health conditions for the pigs. Less deadborn pigs, less deformed piglets, no more pigs that stopped eating.Interesting to see that many of the nasty effects of the GM-fodder is also reported by independent scientists. More on this here:Monsanto's herbicide Roundup linked to birth defects in Argentina's agricultural areas:http://www.monsanto.no/index.p...Â

According to this interesting lecture by professor Gilles-Eric Seralini, Monsanto has not done one proper feeding study with GMO up to 2010. Not one feeding study last longer than 3 months. Not long enough to detect possible chronical and reproductive issues. Then add to this that Monsanto do not report statistical data to the regulators when there is differences in the statistical data between the sexes. But as Seralini points out... this is not valid science, because many of the measured values is already different between the sexes BEFORE the feeding study begins.See the whole lecture here (if you are in a hurry see at least part 4 (10 minutes) of the 5 videos):GMO Risk Assessments Based on Bad Science - You the Guinea Pig:http://www.monsanto.no/index.p...Â

Glyphosate isn't the safe chemical it's cracked up to be. Â There are studies linking it to cancer and other health problems. Â The problem is that glyphosate is thought to break down in the environment, so toxicology studies look at single dose results. Â But glyphosate is very pervasive, and since it is constantly being released, exposure is more chronic than many studies take into account. Â Glyphosate does accumulate in tissue and causes all kinds of problems. Â

Evolutionarily speaking, herbicide resistance is a novel phenomenon.Â Independant, peer-reviewed research on glyphosate is obviously being over-looked by industry boffins, and it's toxological & environmental impact problems' are building, as evidence accumulates.Â Industry heads are ignoring this.Â Glyphosate's heavy mineral chelating effects on the endocrine systems' of all living beings, including soil biota is worth looking into also.Â Glyphopsate does not readily break down, and when it does its metabolite, AMPA, also produces toxic systemic effects.Â High levels of AMPA in water-table is what prompted Denmark to ban this patented chemical.

Are the actual farmers complaining about "superweeds"? No - it's the scientists who are because their "complaints" are really pleas for research grant money in disguise.Â

The actual farmers know the chemical and biotech companies will continue to provide them with effective weed control chemicals for eternity as they always have.Â Have the chemical companies EVER let them down? NO! Have the scientists been screaming "the sky is falling" for decades? YES!

Why do articles like this continue to quote activist groups like the Center for Food Safety and Bill Freese? Â Stick with academic scientists who study the subject, not activists organizations that have an agenda to stop all biotech crops. Â

There have been no long term studies of the effects of eating GM foods spayed with these chemicals (and Monsanto's solution is to spray a second type of herbicide and put even more pollution into our environment). Â

Most unfortunately, the FDA/USDA do not require labeling of GM foods, so any effects can be traced back to their origin. Â Which only protects Monsanto and not the consumers.

Not to mention the underhanded tactics which Monsanto uses enforce their patents to require farmers to use their products, even if the farmer had no intent to use them. Â http://www.mindfully.org/GE/GE...Â

Â A lady went to a doctor once, because a large portion of her forehead was swollen and was blue-black in color.Â He asked her a few questions, including whether her husband had ever physically abused her,Â whether she had fallen and struck her forehead on some piece of furniture or on the floor, whether she had been in an automobile accident...Â In each case, she said she did not recall any such thing having happened to her.Â

Seizing on this repeated answer, the doctor zeroed in on memory issues.Â He asked if she had noticed any memory problems, if she had ever been diagnosed as having epileptic seizures, whether she had ever been knocked unconscious... had amnesia...Â Again, all answers were negative, or were caged as, "Not that I know of."

Trying another tack, he asked her if she had ever been told by anyone that she sometimes fell asleep in mid-sentence and had had awakened and picked up right where she had been in the sentence at time of nodding off.Â Was she taking any prescriptions?Â Any over-the-counter medicines?Â Lots of aspirin?Â

Again and again, her answers were "Not that I know of."Â

So the doctor checked her blood pressure, looked at the capillaries in the backs of her eyes, drew some blood from an arm and had it rushed to the laboratory next door, to check for anything grossly out of the ordinary... iron deficiency, pernicious anemia, liver or kidney malfunction.

The blood results were back after not too long a wait, and nothing was remarkably out of line.

Puzzled, the doctor looked again carefully through his magnifying glass at the lady's forehead.Â He saw no sign of any tumor or cut or infection or anything.Â

By this time that lady was getting impatient and agitated and was fidgeting around.Â She wanted answers, not questions; so finally she said, "I came her to ask YOU what that is on my forehead, so why don't you just TELL me.

The doctor thought for a moment and then said, "All I can see at this time is that you have a big bruise there."

"A BIG BRUISE!" the woman said with resentment.Â "You mean I come all the way to a doctor's office to be told I have a big bruise on my forehead?"

"A subdural hematoma," the doctor replied."

At that the woman shouted, "OF COURSE!" as she whacked her herself on the forehead with the palm of a hand so resoundingly that the "POW" reverberated throughout the clinic.

"WHYÂ DIDN'TÂ YOUÂ JUSTÂ SAYÂ THAT TO BEGIN WITH?"

And, at that, she gathered up her purse and things and departed, with a smile.

To some patients who go to doctors, and to some people for whom evolutionary theory is a narrow, rigid dogmatic view of science and the world, there is only one name you can call something without their taking offense to it, or becoming paranoid as to why you would call it anything else besides what they want to hear.

In the interest of accuracy, the scientist D. Andre' d'Avignon, PhD, who led the team of NMR researchers that discovered the vacuole sequestration mechanism of glyphosate resistance, is employed at Washington University in Saint Louis (NOT at the University of Washington, Seattle).